1. Field of the Invention
The present invention relates to the use of magneto resistive sensors for measuring the diameter of a borehole.
2. Background of the Related Art
Down hole distance and bore hole radius measurements have previously been performed with instruments using potentiometers, linear voltage transformer (LVT) sensors or linear voltage differential transformer (LVDT) sensors. Potentiometers are resistors with an attached position indicating slider. The slider contacts the resistor at any point between the potentiometer ends. The mechanical position of the slider determines the resistance between the slide and the ends of the potentiometer. A position measurement is thereby possible, by measuring either the resistance or the voltage between the slider and one of the potentiometer ends.
In down hole logging instruments, potentiometers are typically placed inside of an oil-filled container, so that the mechanical movement of the slider is not influenced by the bore hole pressure. The oil-filled containers are usually connected to a main pressure housing via a pressure feed through connector and pressure resistant wires. Typically, there is hysteresis between the values measured moving in one direction as opposed to values measured while moving in the other direction. New potentiometer devices exhibit small hysteresis effects but as the devices age, the hysteresis effects becomes more severe as potentiometers begin to wear. Typically, the achievable resolution depends upon the design of the potentiometer. If a wire-wound potentiometer is used (which is usually the case), the resistance between two windings determines the resolution of the wire wound potentiometer.
LVT/LVDT sensors are also popularly used to measure distances downhole. The operating principle behind LVT and LVDT sensors is based on a transformer having a movable ferromagnetic core. The physical position of the core inside of the surrounding windings alters the coupling between the primary and secondary coils of the transformer. While keeping the primary supply voltage constant, the secondary voltage changes in proportion to the position of the positional ferromagnetic core. The LVT sensor uses a single primary coil and one secondary coil. One side of the primary and secondary coils are connected, so that only three wires are necessary to connect an LVT device. The more accurate LVDT device uses two secondary coils. In the LVDT the difference between the two secondary coil voltages is divided by the sum of the two secondary coil voltages, thereby compensating for voltage changes due to unstable supply voltages.
Unlike potentiometer position sensors, LVT and LVDT sensors do not exhibit hysteresis or wear. LVT/LVDT sensors also experience less resolution limitations. Thus, LVT/LVDT sensors are an improvement over potentiometer sensors. The LVT/LVDT measurement, however, is an AC-measurement, thus, the LVT/LVDT sensor signal has to be rectified by some means (hardware, software) for measurement. If larger distances are to be sensed, LVT/LVDT sensors become very large and very expensive. High pressure/temperature versions are possible but they are bulky and expensive. Thus, there is a need for a downhole position indication device that is accurate, durable, compact and not sensitive to down temperature and pressure.
The present invention provides a magneto restrictive position indication device for measuring the radius of a borehole. The present invention does not require any exposed wiring when used in down hole logging equipment. The present invention is also more reliable than known measurement systems because of the absence of hysteresis and because it is less sensitive to downhole pressure and temperature than known systems. The present invention is also easier to maintain. Additionally, the present invention""s measurement accuracy is limited only by the comprising mechanical and electronic components comprising the structure, rather than by the sensor system itself. A measurement accuracy of 50 xcexcm (0.002 inch) is easily achievable with the present invention, and the resolution of the present invention typically is even better by a factor of 5 to 10. The sensor of the present invention also requires less space than all other known measurement systems.
The typical distance measured by the preferred embodiment is preferably 5 mm (just less that xc2xc inch). By combining two of these measurements, distances of up to about 1.5 inches can be reliably measured. Combining three measurements enables distances of up to 10 inches to be measured with the same accuracy and reliability over the full pressure and temperature range downhole.
The structure of the present invention provides a unique signal amplitude between 0xc2x0 and 180xc2x0. In order to obtain unique information over the full 360xc2x0 range, a second sensor (bridge) is provided, which is mechanically shifted by exactly one quarter of the inter pole distance, that is, shifted by 90xc2x0. The output signal of the second sensor also resembles a sine wave, having an amplitude waveform shifted by 90xc2x0 compared to the signal from the first sensor, the cosine.
The tangent or ratio of the sine and cosine is calculated from these two sensor signals. Considering the magnetic pole quadrants for the tangent, this arrangement enables the present invention to determine the exact sensed position of a magnetic ruler over the entire range from 0xc2x0 to 360xc2x0. The resolution and accuracy of the structure of the present invention depends upon the comprising auxiliary hardware, including the resolution of the analog to digital (A/D) converter. Calculating the tangent relationship provides the additional advantage of automatic compensation for temperature drifts and distance changes incurred by magnet/sensor assembly of the present invention.
Preferably, linear magnetic rulers with precisely defined inter pole distances of 0.125 inch to 0.25 inch are provided in order to use the preferred sensors which precisely measure magnetic pole position. Magnetic poles are precisely magnetized or xe2x80x9cwrittenxe2x80x9d on the surface of a magnetic material. Accurately positioned magnetizing devices, which are well known in the art, are used for the production of such precise linear magnets or xe2x80x9cmagnetic rulers.xe2x80x9d
In a preferred embodiment, the arms of the preferred caliper instrument move a sliding or rotating linkage connected to a magnetic ruler and pivoting end of each caliper arm. The caliper arm movement and associated magnetic ruler are sensed by a magnetic resistive sensor, which is contained in a pressure tight housing of the instrument. The present invention provides a pressure tight sensor housing using an o-ring sealed connection to the central housing (which for simplicity is not shown on the drawing). Thus, the measurement instrument does not require any electrical connections exposed to the bore hole fluid.
While one sensor can accurately measure the distance between two magnetic poles. Two or three measurements are combined for determining the absolute position of the magnetic ruler over large position ranges (5xe2x80x3 or more). In a preferred embodiment, the magnetic ruler is in form of a disk or half disk. The magneto resistive sensors determine the angular rotation of the turning disc shaped magnetic ruler as the arm turns. The circular magnetic ruler is attached to the pivoting end of the pivotally attached caliper arm.